In recent times, with the increasing development of information communication technologies, a variety of multimedia services, and a variety of high-quality services have been developed and introduced to the market, so that demands of wireless communication services are rapidly increasing throughout the world. In order to actively cope with the increasing demands, capacity of a communication system must be increased.
A variety of methods for increasing communication capacity under wireless communication have been considered, for example, a method for searching for a new available frequency band in all frequency bands, and a method for increasing efficiency of limited resources. As representative examples of the latter method, a transceiver includes a plurality of antennas to guarantee an additional space utilizing resources so that a diversity gain is acquired, or MIMO communication technologies for increasing transmission capacity by transmitting data via individual antennas in parallel have been developed by many companies or developers.
Particularly, a Multiple-Input Multiple-Output (MIMO) system using an Orthogonal Frequency Division Multiplexing (OFDM) from among the MIMO communication technologies will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a MIMO-OFDM system.
Referring to FIG. 1, in a transmission end, a channel encoder 101 attaches redundant bits to transmission (Tx) data bits to reduce a negative influence of a channel or noise. A mapper 103 converts data bit information into data symbol information. A serial-to-parallel (S/P) converter 105 converts a serial data symbol into a parallel data symbol so that the parallel data symbol can be loaded on several sub-carriers. A MIMO encoder 107 converts the parallel data symbol into space-time signals.
In a reception end, a MIMO decoder 109, a parallel-to-serial (P/S) converter 111, a demapper 113, and a channel decoder 115 have functions opposite to those of the MIMO encoder 107, the S/P converter 105, the mapper 103, and the channel encoder 101 in the transmission end.
The MIMO OFDM system requires a variety of technologies for increasing a Tx reliability of data, for example, a Space-Time Code (STC) or Cyclic Delay Diversity (CDD) scheme to increase a spatial diversity gain, and a BeamForming (BF) or Precoding scheme to increase a Signal-to-Noise Ratio (SNR). In this case, the STC or CDD scheme has been used to increase a Tx reliability of an open-loop system which is incapable of using feedback information at a transmission end, and the BF or Precoding scheme has been used to maximize the SNR using corresponding feedback information of a closed-loop system which is capable of using feedback information at a transmission end.
Particularly, the CDD scheme for increasing the spatial diversity gain and the precoding scheme for increasing the SNR will hereinafter be described in detail.
A Space-Time Code (STC) scheme successively transmits the same signals under the MIMO environment. However, when the STC-based system repeatedly transmits the same signals under the MIMO environment, the same signals are transmitted via other Tx antennas different from the previous Tx antenna, resulting in the implementation of a spatial diversity gain.
The following table 1 exemplarily a variety of space-time codes (STCs) under the STC-based MIMO encoding.
TABLE 1# of Txdp, minSTC SchemeantennaRank RQPSK(1)      1          2        ⁡      [                                        S            1                                                -                          S              2              *                                                                        S            2                                                S            1                                ]  211 (2)      1          2        ⁡      [                                        S            1                                                            S            2                                ]  221 (3)                                                        1                                                2                  ⁢                                      (                                          1                      +                                              r                        2                                                              )                                                                        ⁡                          [                                                                                                                  S                        1                                            +                                              jr                        ·                                                  S                          4                                                                                                                                                                        r                        ·                                                  S                          2                                                                    +                                              S                        3                                                                                                                                                                                S                        2                                            -                                              r                        ·                                                  S                          3                                                                                                                                                                        jr                        ·                                                  S                          1                                                                    +                                              S                        4                                                                                                        ]                                ,                                              r          =                                    5                        ±                          1              2                                            ⁢              220.2 (4)      1    2    ⁡      [                                        S            1                                                S            2                                                S            3                                                S            4                                                            S            2            *                                                -                          S              1              *                                                            S            4            *                                                -                          S              3              *                                                                        S            3                                                -                          S              4                                                            -                          S              1                                                            S            2                                                            S            4            *                                                S            3            *                                                -                          S              2              *                                                            -                          S              1              *                                            ]  414 (5)      1          2        ⁡      [                                        S            1                                                S            2                                    0                          0                                                  -                          S              2              *                                                            S            1            *                                    0                          0                                      0                          0                                      S            3                                                S            4                                                0                          0                                      -                          S              4              *                                                            S            3            *                                ]  411 (6)      1    2    ⁡      [                                        S            1                                                -                          S              2              *                                                            S            3                                                -                          S              6              *                                                                        S            2                                                S            1            *                                                S            6                                                S            5            *                                                            S            3                                                -                          S              4              *                                                            S            7                                                -                          S              8              *                                                                        S            4                                                S            3            *                                                S            8                                                S            7            *                                ]  421
In Table 1, individual rows of each matrix are indicative of individual antennas, and individual columns of the matrix are indicative of time.
FIG. 2 is a conceptual diagram illustrating a data transmission method of a MIMO system using a Cyclic Delay Diversity (CDD) scheme according to one example.
The CDD scheme is as follows. When a system equipped with several Tx antennas transmits the OFDM signal, the CDD system assigns different delays, different levels, or different amplitudes to all the antennas, such that the individual antennas transmit signals with the different delays, levels, or amplitudes. As a result, the reception end can acquire a frequency diversity gain.
OFDM symbols are divided by the S/P converter and the MIMO encoder, and then the divided OFDM symbols are transmitted to individual antennas. Then, the resultant symbols are added to a cyclic Prefix (CP) for the prevention of an inter-channel interference, and the added result is transmitted to the reception end. In this case, although a data sequence transmitted to a first antenna is transmitted to the reception end without any change, another data sequence transmitted to the next antenna (i.e., a second antenna) is cyclically delayed by a predetermined number of samples as compared to the case of the previous antenna (i.e., the first antenna).
FIG. 3 is a conceptual diagram illustrating a data transmission method of a MIMO system using a Cyclic Delay Diversity (CDD) scheme according to another example.
Referring to FIG. 3, if the CDD scheme is implemented in a frequency domain, the cyclic delay may be represented by the product of phase sequences. In other words, as can be seen from FIG. 3, different phase sequences (Phase sequence 1˜Phase sequence M) of individual antennas are multiplied by individual data sequences of the frequency domain, an Inverse Fast Fourier Transform (IFFT) is performed on the multiplied result, and the IFFT result may be transmitted to the reception end. The above-mentioned CDD scheme of FIG. 3 may be referred to as a phase shift diversity scheme.
FIG. 4 is a block diagram illustrating a transmission/reception end of a codebook-based precoding MIMO system.
In the meantime, there are a variety of precoding schemes, i.e., a codebook-based precoding method, and a method for quantizing channel information and feeding back the quantized channel information. In this case, the codebook-based precoding method is used when feedback information is limited in the closed-loop system. Specifically, the codebook-based precoding method feeds back the index of a pre-recognized precoding matrix to the transmission end to get SNR gain.
In this case, each of the transmission/reception ends has a finite precoding matrix (P1˜PL). The reception end feeds back an optimum precoding matrix index (l) to the transmission end using channel information, and the transmission end applies a precoding matrix corresponding to the feedback index to transmission data (χ1˜χMt). For reference, the following Table 1 shows an exemplary codebook used when feedback information of 3 bits is used in an IEEE 802.16e system equipped with two Tx antennas to support a spatial multiplex rate of 2.
TABLE 2MatrixIndex(binary)Column 1Column 20001   0   0   1   0010.7940−0.5801 + j0.1818 −0.5801 + j0.1818−0.79400100.79400.0579 − j0.6051−0.0579 + j0.6051−0.79400110.7941−0.2978 + j0.5298 −0.2978 − j0.5298−0.79411000.79410.6038 − j0.0689 0.6038 + j0.0689−0.79411010.32890.6614 − j0.6740 0.6614 + j0.6740−0.32891100.51120.4754 + j0.7160 0.4754 − j0.7160−0.51121110.3289−0.8779 − j0.3481 −0.8779 − j0.3481−0.3289
The above-mentioned MIMO schemes are requisite for a wireless mobile communication system capable of providing a high transfer rate and a high reliability in a limited frequency band. The above-mentioned MIMO schemes have different performances according to a variety of factors, i.e., a UE (User Equipment) speed, a channel situation, and the number of multi-antennas, such that schemes of different structures must be applied according to channel conditions, resulting in an increase of system complexity and costs.
As can be seen from Table 1, the space-time code (STC) has different codes according to individual antenna structures, and symbols are repeatedly transmitted during several timeslots to get a spatial diversity gain. As a result, the space-time code has a disadvantage in that it unavoidably increases the complexity of the transmission/reception end. Also, the above-mentioned MIMO schemes transmit signals without using feedback information, such that a closed-loop system has a performance less than those of other MIMO schemes using feedback information.
The CDD scheme has a spatial multiplexing rate of 1, such that it is unable to expect the high data transfer date. If resources are fixedly allocated, the CDD scheme has difficulty in acquiring the above gain.
A stable channel is needed for a feedback in a codebook-based precoding scheme because a channel for a UE changes if the UE speed increases. Therefore, the codebook-based precoding scheme is improper for a mobile environment of an abrupt channel variation. Specifically, the codebook-based precoding scheme can be applied to only the closed-loop system.
When the conventional codebook-based precoding scheme is used, uplink transfer rate decreases because each UE needs to feed back codebook index compared with the above-mentioned two MIMO schemes. Also, when the conventional codebook-based precoding scheme is used, the required memory capacity increases and the system complexity increases because each of the transmission/reception ends needs to have a codebook, and because different codebooks are required according to the number of Tx antennas and the special multiplexing rate.